The theory of PDI was already established in the 1970s [1]}, [2]}, [3]}, but its importance in relation to solar-wind generation has continued even in recent studies [4]}, [5]}. The PDI results in forwards (anti-sunward) propagating Alfvén waves decaying into backwards (sunward) propagating Alfvén waves and compressive forwards propagating ion acoustic waves. The forwards-propagating Alfvén wave is often called a `pump wave'. The instability thus has a two-fold importance for solar-wind heating. First, backwards-propagating Alfvén waves can lead to a \(1/f\)  turbulent cascade as they interact with the forwards propagating Alfvén waves; secondly, ion acoustic waves can undergo Landau damping, effectively dissipating energy in MHD scales [6]}. There is observational evidence for the presence of PDI both in the solar wind [7]} and in laboratory conditions [8]}.
